Environment

Double threat of nitrous oxide

Harmful emissions: A farmer harvesting cabbage in a farm in the outskirts of Hanoi, Vietnam. Use of fertiliser in agriculture releases nitrous oxide, both an ozone-depleting and greenhouse gas.

Overlooked but potent, nitrous oxide is a risk to the ozone layer and the global climate.

EMISSIONS of nitrous oxide (N2O), an often overlooked yet potent gas, could nearly double by 2050 and thus potentially undermine gains in the ozone layer recovery and exacerbate climate change.

Drawing Down N2O To Protect Climate And The Ozone Layer, a new report by the UN Environment Programme (UNEP), warns that nitrous oxide is now the most important ozone-depleting emission and the third most potent greenhouse gas released into the atmosphere. While N2O exists naturally in the atmosphere in trace amounts, human activities have increased its concentrations since the industrial revolution.

The UNEP report, produced in conjunction with scientists and experts from more than 35 organisations, points out that reducing N2O emissions has major cost benefits since emissions are connected to diverse economic sectors from agriculture, chemical manufacturing and electricity production to waste management, transportation and fish production. Gains from emissions reduction will include increased crop and livestock productivity, poverty alleviation, improved human health and reduced environmental degradation.

An earlier study quoted by the report indicated that an across-the-board improvement in nitrogen use efficiency of 20% would cost around US$12bil (RM36bil) annually, but would save around US$23bil (RM69bil) in annual fertiliser costs alone. Additional environmental, climate and human benefits could be worth an estimated US$160bil (RM480bil) per annum.

Sewage is processed in an aeration tank in the Shatin Sewage Treatment Works in the New Territories, Hong Kong’s largest sewage treatment facility. Thorough processing of wastewater will prevent emissions of nitrous oxide.

“Although not as prevalent in the atmosphere as CO2 in terms of mass, N2O, commonly known to many as ‘laughing gas’, is far from a laughing matter with respect to climate and ozone damage, as it has a disproportionate impact on global warming because of its radiative properties and long lifetime in the atmosphere, which is 120 years on average. Action on these emissions offer yet another opportunity to keep the world under a 2°C temperature rise,” said UNEP executive director Achim Steiner.

Most of the depletion of the stratospheric ozone layer up to now has been due to the infamous chlorofluorocarbons (CFCs) and other halogenated (chlorine- and bromine-containing) chemicals. However, these chemicals – unlike N2O – are now widely controlled by the Montreal Protocol, an international treaty designed to protect the ozone layer.

Main sources

Agriculture is by far the largest source of human-induced N2O emissions, accounting for two-thirds of these emissions. Meanwhile, other important sources of N2O include industry and fossil fuel combustion, biomass burning and wastewater. The report lists specific, actionable measures that can be taken in each of these areas:

> Agricultural emissions: Emissions can be reduced by boosting the efficiency of nitrogen use in agriculture. This means improving the ability of crops and livestock to better utilise nitrogen, and minimising the loss of nitrogen to the environment that occurs during crop cultivation and animal production. Reducing food waste and loss, as well as consuming less meat – the production of animal protein leads to higher N2O emissions than plant protein – are other options for reducing agricultural emissions of N2O.

> Industrial emissions: The report suggests that significant gains can be achieved by controlling emissions from just two chemical industries – adipic acid and nitric acid – which account for about 5% of global N2O emissions. Such reductions can be achieved by installing emissions control equipment in facilities producing such chemicals.

> Emissions from biomass burning: The report notes that N2O emissions from landscape fires can be achieved by reducing the use of fires for forest clearing and implementing prescribed burning to reduce the amount of burnable vegetation in natural fires. Improving the fuel and combustion efficiency of stoves can reduce emissions from biomass-burning in household stoves.

> Emissions from wastewater and aquaculture: Proper collection and treatment of wastewater can reduce N2O emissions to the atmosphere. These include improved wastewater treatment, reducing wastewater leakage from sewage piping, and recycling nutrients in wastewater as fertiliser. Emissions from aquaculture can be lowered by boosting overall nitrogen use efficiency, and by implementing fish farming systems that reduce the amount of waste generated, or by treating effluents from fish ponds.

Overcoming barriers

Cost, capacity building, technology transfer, and the lack of know-how are among the barriers to implementing N2O reduction strategies at a global scale.

Possible actions to overcome such barriers include: removing subsidies that encourage the overuse or misuse of nitrogen fertiliser while providing incentives for adopting best management practices that would improve nitrogen use efficiency; putting a price tag on nitrogen pollution through appropriate levies, incentives and tradable permits; encouraging research and development in techniques that enhance nitrogen use efficiency and crop productivity; supporting good nutrient management practices in crop and livestock operations; and setting targets for N2O emission reductions and putting in place strategies for tracking progress.

Reducing N2O emissions will also help protect the ozone layer. Rising levels of N2O may undermine the gains in ozone layer recovery achieved by drawing down CFC and other ozone-depleting substances. Reducing emissions will help avoid the continued depletion of the ozone layer and secure the gains made by the Montreal Protocol. – UNEP